Low Cost Wafer Bumping of GaAs Wafers

The microelectronics industry has implemented a significant number of process technologies to accomplish the various packaging and backend operations. These technologies have been successfully implemented at a number of contract manufacturing companies and also licensed to many of the semiconductor manufacturers and foundries. The largest production volumes for these technologies are for silicon based semiconductors which are based on either aluminum or copper interconnect metallurgy. The direct transfer of these technologies to compound semiconductor devices, like GaAs, LiTaO3, and GaN, is limited due to a number of technical compatibility issues and several perceived compatibility issues [1-4]. From a technical standpoint, many of these high end devices contain fragile air bridges, gold bond pads, cavities & trenches, and unique bulk material properties which are sensitive to many of the mechanical and chemical processes associated with many of the standard packaging operations using for silicon wafers. Special care must be taken to ensure that there is no mechanical stress put on the wafer during any of the handling operations associated with deposition of the UBM, solder bumping, wafer thinning, dicing, and die sort. In addition, many chemicals used for resist stripping, metal etching, and solder fluxing will react with some of the materials on these compound semiconductor devices. From a perception standpoint, companies which are processing large numbers of silicon based semiconductor wafers in their packaging and backend facilities, are reluctant to process many of these compound semiconductors because there is a perceived issue with cross contamination between the different wafer materials. Companies are not willing to risk their current business of processing silicon wafers by introducing these new materials into their existing process flow. The strategy in this study is to protect all structures and surfaces with a resist or film as part of each step in the process. This protects the wafer from mechanical and chemical damage; and at the same time protects sensitive fab processes from contamination by the compound semiconductor. Introduction The test vehicle for this program is a GaAs device which contains both air-bridges and has gold bond pads. If solder were deposited directly onto the gold bond pads, the gold would be partially consumed during the solder reflow process, and will eventually fail in the field due to thermal and electro-migration processes. An under-bumpmetallization (UBM) is required to keep these destructive processes form occurring. Schematic drawing of GaAs test vehicle and an optical image of a gold bond pad on the device. Air Bridge Bond Pad Dicing Street